A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
A projection lithography apparatus uses a projection optical system to project an image of the pattern onto the substrate. The image projected onto the substrate, and hence the device manufactured, is sensitive to various forms of error that may be caused by aberrations in the projection system. Since the projection system is inevitably not 100% transmissive and the power level of the projection beam is high in order to provide short exposure times and hence high throughput, the projection system absorbs a significant amount of energy from the projection beam and heats up. In spite of highly effective cooling systems, heating of elements of the projection system is sufficient to distort them and introduction of aberrations can occur. This phenomenon is often referred to as lens heating. Illumination modes are usually described in terms of the intensity distribution of the radiation beam in a pupil plane of the projection system. Common illumination modes include: conventional illumination, in which the radiation beam is contained uniformly in a central disc in the pupil plane; dipole illumination, in which the radiation beam is contained in two poles located away from the optical axis of the illuminator; annular, in which the radiation beam is contained in an annulus concentric to the optical axis; and quadrupole illumination, in which the beam is contained in four off-axis poles. Dipole and quadrupole illumination modes in particular result in strong localization of the radiation beam in the projection system and hence to localized heating.
It is known to provide adjustable elements in a projection system to compensate for aberrations caused by lens heating, which can in many cases be predicted using software. Aberrations in an optical system are often described in terms of Zernike polynomials which are a set of orthogonal basis functions particularly useful to describing functions having some degree of rotational symmetry. It is therefore known to provide adjustable elements that can effect an adjustment affecting aberrations.
However, existing arrangements for compensating for aberrations induced by lens heating are not particularly effective when used with strongly localized illumination modes such as dipole and conventional illumination with small sigma (pupil filling) values.